Saturated or unsaturated aliphatic ketones or alcohols, such as methyl ethyl ketone (MEK), methyl vinyl ketone(MVK), cyclohexanone and cyclohexanol, are important chemicals used in various applications in the chemical and pharmaceutical industries. They are important intermediates and are among the most preferred organic solvents or reagents used in producing other more valuable specialty chemicals or pharmaceutical products. For example, MEK is a stable and low viscosity aliphatic ketone partially miscible in water while completely miscible with most organic solvents. This exceptional solvency makes MEK the second most important (next to acetone) commercially produced ketone for use as an organic solvent in various industrial applications, including coating and paints, adhesives, tapes, and lube oil de-waxing, etc. As another example, MVK is an effective alkylating agent and a useful intermediate with applications in organic synthesis, including the syntheses of pharmaceutical products such as vitamins or steroids. By way of further example, cyclohexanone, in addition to serving as a common organic solvent and reagent, is used as the precursor for making caprolactam, the monomer used for the production of Nylon-6. By way of yet another example, cyclohexanol is the alcohol component of KA oil, a mixture of ketone cyclohexanone and alcohol cyclohexanol. KA oil is the key intermediate for the production of both Nylon-6 and Nylon-6,6.
With respect to the conventional manufacturing of MEK, the most widely used commercial process is a three-step sec-butyl alcohol process, commonly known as the SBA process, which starts with the oxidation of 1-butene in sulfuric acid followed by hydrolysis, acid stripping, neutralization and separation leading to sec-butyl alcohol, which subsequently undergoes gas phase catalytic dehydrogenation with oxides of Cu, Zn or Cr as the catalysts to produce MEK [Ullmann's Encyclopedia of Industrial Chemistry, (2001) 6th Ed.]. There are many drawbacks of the SBA process, mainly relating to the higher manufacturing cost associated with the use of 1-butene as the starting material; the usage and recycling of a large quantity of corrosive sulfuric acid; the multiple steps of oxidation, hydrolysis, neutralization and separation as well as the treatment of a large quantity of acid sludge and other toxic waste generated in the process. Beside the economic drawbacks, the environmental pollution that results from the large quantity of toxic waste generated is very serious, while the energy consumption required for all the production and cleanup steps is substantial. Another commonly used commercial process for the production of MEK is the liquid phase oxidation of n-butane. In this liquid phase process, acetic acid is used as solvent, cobalt and sodium acetates are the homogeneous catalysts, under which condition, n-butane is oxidized by air in the liquid phase to MEK. Although this liquid phase process has been practiced for more than half a century, it is not efficient for MEK production because most of the MEK thus formed is further oxidized to acetic acid and other by-products, due to the difficulty in preventing such further oxidation in the process.
With respect to the conventional manufacturing process for cyclohexanone, cyclohexanol, or the corresponding ketone-alcohol mixture KA oil, the two most used processes are the liquid phase cyclohexane oxidation and phenol hydrogenation. The classical liquid-phase cyclohexane process, developed in the 1940s, is still a preferred process for the industrial production of KA-oil today [Industrial Organic Chemistry, Wiley-VCH Press, (2003) 4th Ed]. In the liquid phase at about 150° C. under high pressure, cyclohexane is oxidized to KA-oil with cobalt salts as the catalysts. Typically, the classical process achieves around 4% cyclohexane conversion and 3.4% yield of KA oil. To improve the extremely low efficiency of this classical liquid-phase process, a modified process involving boric acid was developed in the 1950s. The boric-acid modified process increased the one-pass conversion of cyclohexane somewhat to around 10% and the KA yield to about 9%. This improved conversion rate, however, is achieved at the expense of serious pollution and increased operation costs, as “large amount of solids [wastes] need to be separated and decomposed and boric acid has to be recycled”. The conventional liquid-phase cyclohexane process has since been characterized as “the least efficient of all major industrial chemical processes” [U. Schuchardt et al, Synlett 10 (1993) 713]. At the present time, the main drawbacks for liquid phase cyclohexane oxidation are still the low efficiency, pollution and high energy consumption.
The economic incentive to overcome such low efficiency is huge, and has continuously driven research and development efforts worldwide. The majority of such R&D efforts are concerned with modifications of the catalyst system with respect to a wide spectrum of factors ranging from metal element, oxidation state, morphology, chelating agents for organometallic complexes, host or support materials, to catalyst preparation method, etc. Similarly, exploring alternative non-air oxidants, such as hydrogen peroxide and tert-butyl hydroperoxide, has continued to be a topic of interest for many researchers. However, none of the modifications of the liquid-phase process have thus far achieved any higher efficiency than the boric-acid modification, and all of these modifications usually come with a set of new problems. Indeed, it seems that the conclusion by the recent review [U. Schuchardt et. al, Appl. CataL A. Gen, 211 (2001) 1] is still fairly accurate that “cyclohexane oxidation [in liquid phase] continues to be a challenge”.
In contrast to the extensive research and development efforts devoted to improving the liquid-phase catalytic process for the production of corresponding ketones from alkanes, very few publications have appeared concerning the catalytic production of aliphatic ketones by gas phase oxidation of alkanes. While gas-phase catalytic oxidation of n-butane over VPO catalysts has been well studied and was successfully commercialized in the 1980s, it is for the production of maleic anhydride, and not for the production of any ketones [N. Ballarini, et al, Topics in Catalysis, 38 (2006) 147]. Insofar as is known, no publication to date has described the production of a measurable amount of MEK from n-butane catalytic reaction in the gas phase. Likewise, very few publications have appeared concerning the production of cyclohexanone by gas-phase catalytic oxidation of cyclohexane.
U.S. Pat. No. 2,386,372 to Wagner is directed to solid catalysts of metal or metal oxides of Ag, Cr, Cu, Fe, V etc. for the oxidation of cyclohexane to cyclohexanone. The actual example described therein is the oxidation of methylcyclohexane. However, extensive follow-up research on cyclohexane gas-phase catalytic oxidation over 11 solid catalysts, including most of the catalysts claimed in U.S. Pat. No. 2,386,372, revealed that CO2 and water were the only products found [W. Hoot and K. Kobe, J. Ind. & Eng. Chem., 47 (1955) 776]. It was only in recent years that any further attempts at gas phase catalytic oxidation of cyclohexane were reported, such as those over Zn—Cr—O catalyst [F. Patcas et al, Progress in Catalysis, 8 (1999) 54], over several other oxide catalysts containing transition metals V, Mn, Ni, Cu, Zn and Mo etc. [C. Hettige, et al, Chemosphere, 43 (2001) 1079], and over CuOx oxides supported on SiO2 and fiberglass [J. Medina-Valtierra et al, Appl. Cat. A, 238 (2003) 1]. Except for the cyclohexane oxidation over the supported CuOx catalysts (wherein cyclohexanone and cyclohexanol were reported among many other products), all of the other above-mentioned attempts confirmed the early conclusion by Hoot and Kobe that a) CO2 was the main product, and b) no cyclohexanone or cyclohexanol were detected in the product streams. The present inventors attempted to reproduce the oxidation of cyclohexane over the CuOx catalyst supported on SiO2. However, benzene and CO2 were the only products detected, while no cyclohexanone could be found when cyclohexane was subjected to gas phase oxidation over the CuOx/SiO2 catalyst (comparative example-5). The present inventors also investigated a silica supported Au catalyst (comparative example-6) which is said to catalyze the selective oxidation of cyclohexane to cyclohexanone in the liquid phase [K. Zhu et al, Catal. Letter. 100 (2005) 195]. This supported gold catalyst, Au/SiO2, was found to be slightly different from the CuOx/SiO2 catalyst in that CO2 was the only product detected from the cyclohexane gas phase oxidation.
Hence, there is a longstanding need for an innovative process involving a novel catalyst useful for the production of aliphatic ketones from heterogeneous catalytic oxidation of C3-C9 alkanes in the gas phase.